1. Field of the Invention
The present invention relates to snubbers for use on piping and components of power plants and processing facilities and, more particularly, to an improved seismic stop support which limits the dynamic motion of the supported structure.
2. Description of the Prior Art
During the 1970s, an increasing concern was expressed by the U.S. Atomic Energy Commission and the U.S. Nuclear Regulatory Commission personnel regarding the reliability and safety of nuclear power plant piping under severe dynamic loads caused by fluid transients or seismic activity. Attention was therefore directed to analyzing the design and safety of the piping and components, and extensive testing programs were undertaken.
Two conflicting requirements characterize the design of high temperature piping. First, in order to minimize thermal stresses, the pipe supports must permit pipe movement from rising thermal expansion to be relatively unrestrained. Second, in order to minimize dynamic stresses, the supports are required to restrain the dynamic motion of the pipe in the event of a dynamic excitation such as an earthquake. The need to resolve this conflict of function in the pipe supports led to the development of snubbers.
Snubbers permit pipe displacement resulting from slow movements, such as thermal expansion, but they restrain rapid motion, such as that induced by earthquakes. They are considered active supports because their performance is response dependent and rely on moving components or pressurized medium in the snubbers. The snubbers change function in response to dynamic motion excitation of thermal piping. The two basic types of snubbers are hydraulic and mechanical. Either type moves freely to accommodate thermal movement of piping and locks up if the piping or its components are subjected to rapid movement and high amplitudes caused by a seismic event or dynamic load such as water hammer.
The problems associated with the hydraulic and mechanical snubbers have been documented and set forth in a Nuclear Regulatory Commission circular entitled, Aging and Service Wear of Hydraulic and Mechanical Snubbers Used on Safety-Related Piping and Components of Nuclear Power Plants--Phase I Study--which is NUREC/CR-4279; PNL-5479; Vol. 1 (RM, RV), prepared by S. H. Bush, P. G. Heasler, R. E. Dodge of the Pacific Northwest Laboratory, Richland, Wash. 99352, completed January, 1986, and published February, 1986. It was prepared for the Division of Engineering Technology, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C. 20055, NRC FIN B2865. A copy of that circular is filed with the patent application for the present invention and made a part of the record hereof.
The goal of the seismic design procedures was to maintain piping frequencies sufficiently above the dominant excitation frequencies such that piping response was not significantly amplified. This can be readily accomplished by relying on snubbers to restrain the dynamic loads. The ease with which snubbers can be employed in analysis, and the demands of plant design and construction deadlines, have resulted in a proliferation of snubbers in nuclear power plant piping.
Although convenient for purposes of design and analysis, snubbers have many disadvantages that become apparent in the course of plant construction and operation. Both mechanical and hydraulic snubbers are sensitive devices that require special care in installation. Careless handling, forced alignment of the spherical bearings, and arc strikes from negligent welding have resulted in malfunctions. Problems arising from leaking fluid, degradation of seal material, and corrosion have necessitated frequent in-service inspection and testing at all nuclear power plants.
Because many snubbers are located in areas of high radiation, exposure of personnel to radiation during maintenance is of concern. Many recently built nuclear power plants use as many as 2,000 snubbers, costing anywhere from $1,000 to $15,000 each. Maintenance costs for a single snubber in the lifetime of a plant may reach $60,000. In view of these problems, considerable advantages in cost and reliability could be gained by eliminating or reducing the use of snubbers. However, some form of restraint is necessary to restrict large dynamic displacement and maintain piping stresses at acceptable levels. Any such restraints must be employed as easily as snubbers in the piping system design and analysis.